I think you're just reading too much into the problem. The terminology can be a bit daunting and it can be very hard for bioinformatics people to realize just how much specialist terminology we end up using. I mean, we have both computer science and molecular biology jargon that we use every day. It's sometimes hard to recognize just how much assumed knowledge there is in this field.
Unfortunately, bioinformatics is full of edge cases like this.
In this example, if you're talking about the size of a full protein, you need to add the mass of a water (to account for the lack of a bond at the N and C terminus). However, if you're talking about the size of a peptide fragment (a sub-sequence), then you don't need to account for the extra water molecule [0]. And as the explanation says -- peptide identification is the most common use-case for this analysis, so we'll just use that. This is also why the table lists the "residue" weights -- because it's easier to calculate the `sum(residue_weight) + water` as opposed to `sum(amino_acid_weight) - (n-1) * water`.
I think the extra explanation isn't entirely clear, but I wouldn't get hung up on that part or let that bother you too much. The problem really boils down to: calculate a weighted sum of a string using these weights for the letters. That's it... no more, no less.
Congrats on working through the exercise. This is honestly great resource for learning about the field from a coding perspective. As others have mentioned, this is from the people who literally wrote the book about much of the field.
[0] Note: In the protein, you aren't actually adding an intact water molecule itself, but accounting for the extra H at one end, and an extra OH at the other end. Which is the same weight as HOH, or 18.01056 Da.
I think it means that I don't want to work in bioinformatics, if there's such ambiguity over what seems to be a basic term.
Does the term "residue" include the ex-amino acid components at the end of the protein?
The text is clear that it doesn't. The popup says that "a residue is a molecule from which a water molecule has been removed". But it's internally inconsistent, and the IUPAC definition says that it does include the ends.
As to whether you might consider working in the field of bioinformatics, I can only pass on a comment from a researcher in the field. She says that they are being "swamped with data". Lots of data, but a lack of computer nerds to work on it.
PS I also noticed the reference to post-translational modification in one of your comments. This is one of the factors that makes for the amazing variety in protein/enzyme functioning. Humans have about 20 000 genes to encode about 75 000 different enzymes, plus a multitude of structural, storage, hormonal, etc. proteins. So you can see that translation of the gene is only the beginning of the story; post-translational modifications are needed. https://www.sciencedirect.com/topics/neuroscience/posttransl... It's an unlimited area of study - one I stepped out of and wish I had stayed in. Of course, to make any sense of a complex system, one has to concentrate on a small part of it. Focus on what takes your interest (and you can get paid for).
I don't know if it's true, but when I see "Lots of data, but a lack of computer nerds to work on it." ... I read it as saying there's no money to hire those computer nerds.
The same person advised me that the software developers who go into science often do so for the love of the topic, and willing to be paid less. I don't think I can get that interested in bioinformatics.
Yes, biology is a mess of exceptions, "good enough" and "hey, it works". It does have its charm as well, though.
One of the first things I learned from that world is that if you ask a microbiologist if something is always the case, the answer will be "Yes" if it is true more than ~80% of the time.
For amino acids, the interesting thing is that they get joined into chains that fold up into proteins (which do all the work). The residues after that happens looks like this:
R O R O
H | || H | || H
----- N - C - C --------- N - C - C ----------- N etc
H H
When the amino acids are isolated (not chained) the double-bonded carbon on the right carries an extra OH group. The whole right-hand carbon is often written COOH. The nitrogen on the left hand side of each amino acid carries an extra H. As part of the process that joins the amino acids together, the OH on the left and the H on the right pair off to form water. This is called condensation, and it happens at every junction between two acids. So if there are n amino acids, there are n-1 junctions and n-1 water molecules that were present (in aggregate) in the constituent amino acids that don't make it into the final protein chain.
Note, though, that one of the methods of chopping up protein chains is adding the water molecules back again, so you should know exactly what you're looking if you want to count masses precisely.
Jeez. Didn't mean to write all that. Hope it helps.
Rather nit-picky, though. Thanks for the diagrams :)
Let's suppose the amino acids HO-A-H, HO-B-H and HO-C-H came together (I hope I got the sides correct!) to form HO-A-B-C-H.
Are HO-A- and -C-H "residues"? One of them lost an -H and the other an HO-, so they are "what's left over after whatever was going to happen has finished happening."
The text says as "every amino acid in a protein are residues except the leftmost and the rightmost ones", so those end ones are not residues.
But the IUPAC definition says it can lose one, or the other, or both, which would include the end ones as part of the definition.
(The key is that residues are created whenever a molecule of water is expelled. In theory, you can expel exactly one water molecule from each amino acid to create a residue, with one hydrogen from the amine group and the hydroxl from the carboxylic acid. In reality, it takes two amino acids to start the process of peptide synthesis, and each aa contributes only one part (either the proton or the hydroxl group) of the theoretically expelled water molecule. In reality its not even a single water molecule, as the hydroxyl group is going to pick up a spare proton from solution and the extra proton on the amine will get pick up by gawd knows what) <-- Downright wrong (at least, in regards to its importance to residues.).
The condensation is connected in this instance but not in all instances of residues. I think the usage is more clear if you look at some of the other examples in the biochemistry section of the wiki page "...a residue refers to a specific monomer within the polymeric chain of a polysaccharide, protein or nucleic acid. One might say, "This protein consists of 118 amino acid residues" or "The histidine residue is considered to be basic because it contains an imidazole ring." (https://en.wikipedia.org/wiki/Residue_(chemistry))
The first paragraph of the expanded question text has: "every pair of adjacent amino acids has lost one molecule of water, meaning that a polypeptide containing n amino acids has had n−1 water molecules removed"
The second paragraph has: "Thus, the mass of a protein is the sum of masses of all its residues plus the mass of a single water molecule."
The fifth paragraph has: "The mass of a protein is the sum of the monoisotopic masses of its amino acid residues plus the mass of a single water molecule"
And the monoisotopic mass table says "Note: the monoisotopic mass of water is considered to be 18.01056 Da."
So I thought that the water molecule was important in the calculation.
However, the last paragraph (which I only now closely read) says it isn't important, with "In the following several problems on applications of mass spectrometry, we avoid the complication of having to distinguish between residues and non-residues by only considering peptides excised from the middle of the protein. This is a relatively safe assumption because in practice, peptide analysis is often performed in tandem mass spectrometry."
Since it didn't mention "water", and instead used the specialist term "residue", I missed the connection earlier.
That said, the text seems to use "residue" inconsistently. There's the definition "a residue is a molecule from which a water molecule has been removed; every amino acid in a protein are residues except the leftmost and the rightmost ones."
but there's also the usage: "the mass of a protein is the sum of masses of all its residues plus the mass of a single water molecule"
Surely that should be "the mass of a protein is the sum of masses of all its residues plus the mass of its leftmost and rightmost amino acids minus the mass of a single water molecule", yes?
So I looked up the definition of "amino acid residue". It appears to be https://goldbook.iupac.org/terms/view/A00279 "α-Amino-acid residues are therefore structures that lack a hydrogen atom of the amino group (–NH–CHR–COOH), or the hydroxyl moiety of the carboxyl group (NH2–CHR–CO–), or both (–NH–CHR–COO–); all units of a peptide chain are therefore amino-acid residues".
https://en.wikipedia.org/wiki/Protein_sequencing#Whole-mass_... also agrees that "residue" includes the two amino acids at the ends, saying "The protein’s whole mass is the sum of the masses of its amino-acid residues plus the mass of a water molecule and adjusted for any post-translational modifications"
Which means ... I don't think the author uses the term "residue" correctly?
Or, more likely, I'm confused by the specialist terminology. Can someone clear up my confusion?